Precision electrical resistor



Nov. 1, 1966 w. R. JOHNSTON ETAL 3,282,730

PRECISION ELECTRICAL RESIS'I-OR Filed Nov. 14, 1962 United States Patent O 3,282,730 PRECISION ELECTRICAL RESISTOR William R. Johnston and Howard A. Cramer, Independence, Kans., assignors to Electra Manufacturing Company, Kansas City, Mo., a corporation of Missouri Filed Nov. 14, 1962, Ser. No. 237,557 2 Claims. (Cl. 117-212) The present invention relates generally to electrical resistors and methods for making the same. It more particularly concerns a precision resistor having a body formed of a unicrystalline substrate material.

Rocket, missile and satellite technology has intensified the need for high-precision components of all kinds. Large numbers of precision electrical resistors are required both in the ground control systems and in the missiles themselves. Due to the extremely high prelaunching costs involved, it is essential that such components have near absolute reliability, in addition to meeting severe size and weight limitations.

Conductive film-type resistors have been widely accepted for such precision applications employing either a carbon film or a metallic film as the conductive medium. Using the vapor deposition processthe conductive composition and amount of the coating may be precisely controlled. In this regard, reference is made to copending application Serial Number 165,505, filed January l1, 1962, and now Patent No. 3,190,771 by William E. McLean and Gaylord A. Swartz, also assigned to the assignee of the present invention. Substrate materials include various glasses, ceramics, steatite, and sintered alumina. However, even when the highest purity centered or multicrystalline material is used, defects due to imperfections and physical faults cannot be completely controlled. It is found that the increased current density which occurs in the vicinity of the impurities or faults tends to produce localized hot spots causing decomposition of the conductive lm and changes the electrical characteristics of the resistor, setting the stage for catastrophic failure. In most cases these faults cannot be detected without destructive or time consuming tests.

Accordingly, it is the primary aim of the present invention to provide a precision conductive film-type resistor having a unicrystalline substrate which is free from physical faults and surface defects to insure substantially perfect reliability. It is also an object of the present invention to employ such a unicrystalline substrate for use in a precision electrical resistor in which any faults or defects in the substrate material may be readily detected by optical examination prior to the fabrication of the resistor.

It is a related object to provide such a unicrystalline substrate which has a high thermal conductivity so that heat is conducted away from the resistive film at a uniform and rapid rate which permits a substantial reduction in resistor size.

An additional object of the present invention is to provide a precision resistor of the above type having a unicrystalline substrate of great hardness, the surface of which may be highly polished to provide a smooth and flawless surface for the conductive film but which may be formed in any convenient shape such as rods or wafers.

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

FIGURE 1 is a perspective view of a precision electrical resistor embodying the present invention;

FIG. 2 is a section taken along the line 2-2 in FIG. 1; and

FIG. 3 is a perspective view of another embodiment of a precision resistor also constructed in accordance with the present invention.

While the invention will be described in connection with a certain preferred embodiment and procedure, it will be understood that we do not intend to limit the invention to that embodiment and procedure. On the contrary, we intend to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

Turning now to the drawings, there is shown in FIG- URE l a precision resistor 10 embodying the present invention. In the illustrated embodiment, the resistor 10 has an elongated cylindrical body 11 to which a thin coating of conductive film 12 has been applied. The cylindrical body 11 has been provided with metallic end caps 13 on each end, and an electrical lead or terminal 14 is suitably connected to each end cap. In order to protect the surface of the conductive film 12, the entire resistor body 11 and end caps 13 may be covered with a protective coating (not shown).

In accordance with the present invention, the resistor body 11 is formed of a unicrystalline material which has a highly polished surface on which the conducted film 12 is applied and which, in the preferred embodiment, is formed of a synthetic gemstone from the corundum family, such as synthetic sapphire. As shown in FIGS. 1 and 2, the resistor `body 11 is conveniently a rod of single synthetic sapphire crystal.

Synthetic sapphire is manufactured commercially by the Verneuil or flame fusion process in which chemically prepared, fine particle, high purity alumina (A1203) is dispersed into the center tube of an oxygen-hydrogen blow pipe. The particles melt at 2,040 C. and fuse on a seed rod, which is slowly withdraw from the furnace. As additional material is fed to the growing crystal or boule, the crystal is lowered to maintain a molten cap in the same region of the flame. In this way it is possible to produce small diameter sapphire rods up to 2O inches long. These rods may then be ground to a uniform diameter and cut to length for individual resistor bodies. Alternatively, unicrystalline sapphire discs may 'be formed by growing and subsequently slicing a crystal of greater thickness to provide a `thin wafer shaped crystal.

Unicrystalline synthetic sapphire is optically transparent, with transmission in the visible range. Therefore, the crystal can be subjected to optical inspection for both surface and sub-surface faults. Moreover, because this optical inspection is not destructive, the tested material is fully useable after inspection. In this way resistor bodies made of this unicrystalline substrate can be efficiently examined and only perfect crystals chosen for precision resistors.

The thermal conductivity of synthetic sapphire has been found lto be about three times greater than the best grade of sintered alumina substrate material and ten times greater than dense steatite. Thus, it is possible to reduce the size of the substrate material for conductive film-type resistors since the higher thermal conductivity rating allows the unicrystalline substrate material to dissipate heat from the conductive film much more rapidly. As a practical example, tests have proven that up to a 6.5 to 1 reduction in resistor size can be effected by the use of synthetic sapphire instead of dense steatite material.

It is another feature of the present invention that a resistor body 11 made of synthetic sapphire can be finished with extremely smooth polished surface. Preferably, the irregularities in the surface are less than about 20 to 30 micro-inches per inch. This feature is due to the extreme hardness and inherent purity of the unicry-stalline sapphire. In this regard, sapphire has a hardness of Moh 9 which is exceeded only by that of diamond which has a hardness of Moh l0. This hardness not only permits the highly polished finish for the resistor body, but also pra-ctically insures that once the surface has been polished it will not become subsequently scratched. Therefore, a resistor body of synthetic sapphire may be made without the slightest surface defects-and, as mentioned above, any defects which may rarely occur can be readily detected by optical inspection. f

In addition to the hardness and optical properties of synthetic Sapphires, they are also uniquely suited for resistor substrate applications because of their high melting point (2040 C.) and dielectric properties. Furthermore, the electrical resistance of synthetic sapphire is very high, up to 100,000 megohms per centimeter at 500 C. While the thermal shock characteristics of synthetic Sapphires depend somewhat upon the orientation f the optical axis, properly manufactured sapphire crystals can withstand both high temperatures and high thermal shocks. In this regard, these anistropic crystals, as grown, are commonly furnished with the optical axis about 60 from the major geometrical axis, although other orientations are available.

Synthetic sapphire crystals also have good mechanical strength which insures that lsapphire resistor bodies 11 can withstand rugged physical treatment. The compressive strength of sapphire is 300,000 p.s.i. and the modules of rupture ranges between about 50,000 p.s.i. to about 100,000 p.s.i. depending on the orientation of the optic axis. Also, since unicrystalline sapphirehas zero porosity there is no problem with outgassing during or subsequent to the coating process which would diminish the uniformity of the conductive coating 12.

The resistivity of the resistor is dependent upon the amount and characteristics of the conductive lm 12 which is applied to the surface of the unicrystalline substrate 11. It will be understood that the conductive film 12 may be composed of carbon formed in situ, a metal, or a metal alloy without departing from the present invention. In the preferred embodiment, the conductive lm is a nickel-chromium alloy applied to the substrate by the vapor deposition process. It has been found that a good mechanical bond can be made between the highly polished surface of the substrate material and the metallic or carbon film. p

Using the vapor deposition process it is possible to control the precise composition of the alloy which is used to coat the substrate material. Moreover, this process permits the amount of metallic film to be controlled within closely defined limitations. Thus, it is possible to manufacture a precision resistor having an electrically conductive lm 12 with a perfectly uniform high resistance value from one point on the resistor body to another.

Turning now to FIGURE 3 there is shown another embodiment of a precision electrical resistor constructed in accordance with the present invention. In this embodiment, the resistor 20 includes a wafer-like b-ody 21 formed from a substantially flat unic-rystalline substrate material having `at least one highly polished face 22. 'Iihe conductive film, in this case, is deposited 0n the face 22 in the form of a relatively t-hin, narrow path 23 having end terminals 24 at opposite edges of the body 21. While the conductive path 23 may ybe applied in `any suitable manner, it will be appreciated that the face 2 2 of the body 21 m-ay be entirely coated, such as bythe vapor deposition process described above, Iand the excess portion of the conductive film removed, for example by etching, to provide the conduct-ive path 23. It wil-1 be further appreciated that resist-ors 20 having a wafer-like conliguration also possess the advantageous features arising from t-he use of a unicrystalline substrate .as discussed above. In this instance, the tiat face 22 may be highly polished and the wafer 21 subjected to opt-ical inspection in order to detect any surface or sub-surface faults 'before applying the conductive material 23.

Precision resistors constructed in acco-rdance with the teachings of the present invention have a substantially perfect reliability rating.

While the use of synthetic sapphire has been particularly mentioned for use in precision electrical resistors, it will be appreciated that other unicrystalline materials ranging in hardness from Moh 7 to 9 may also 'be used for the resistor bodies. Unicrystalline spinel, for example, another synthetic gemstone, may be advantageously employed without departing from the present invention. Spinel is Ialso a Very hard transparent crystal (Moh 8) which can be highly polished and optically inspected for surface and sub-surface faults. The other properties, including strength, porosity, dielectric constant `and melting point of spinel and other synthetic gemstones make them particularly suitable as a substrate material for precision resistors.

We claim as our invention:

1. An electrical resistor comprising, in combination, a resistor substrate formed of a rod of synthetic sapphire, said rod 4being ground to a uniform diameter with a highly polished surface having surface irregularities of less than about 20 to 30 micro-inches per inch, and a vapo-r deposited film of nickel-chromium alloy applied on said highly polished surface of said rod to define a path of high electrical resistance on said resistor.

2. An electrical resistor comprising, in combination, a resistor substrate formed of .a Wafer-like slice of synthetic sapphire, said wafer-like slice having `at least one smoothly polished face, ground to a uniform finish having surface irregularities of less than :about 20 -to 30 microinches per inch, .and an electrically conductive iilm of nickel-chromium alloy applied by the vapor deposition process -to said polished face of said Wafer-like slice to dene Ia conductive path of high electrical resistance on said resistor.

References Cited by the Examiner UNITED STATES PATENTS 2,43 0,581 11/ 1947 Pessel 117-227 2,608,031 8/1952 Barnes et al. 323--142 2,950,996 8/ 1960 Place et al 117-227 2,968,866 1/1961 Soper et al. 117-4 2,993,815 7/1961 Treptow 338--308 XR 2,997,979 8/1961 Tassara 117--227 3,015,587 1/1962 MacDonald 117-227 ALFRED L. LEAVI'IT, Primary Examiner.

RICHARD D. NEVIUS, Examiner.

W. L. JARVIS, Assistant Examiner, 

1. AN ELECTRICAL RESISTOR COMPRISING, IN COMBINATION, A RESISTOR SUBSTRATE FORMED OF A ROD OF SYNTHETIC SAPPHIRE, SAID ROD BEING GROUND TO A UNIFORM DIAMETER WITH A HIGHLY POLISHED SURFACE HAVING SURFACE IRRGULARITIES OF LESS THAN ABOUT 20 TO 30 MICRO-INCHES PER INCH, AND A VAPOR DEPOSITED FILM OF NICKEL-CHROMIUM ALLOY APPLIED ON SAID HIGHLY POLISHED SURFACE OF SAID ROD TO DEFINE A PATH OF HIGH ELECTRICAL RESISTANCE ON SAID RESISTOR. 